1. Field of the Invention
The present invention relates to a wrist-mounted-type antenna device that is mounted on a wrist-mounted-type apparatus that transmits and/or receives signals, such as a pager, a TV, a receiver, a transmitter, a radio, for example. In particular, the invention relates to improvements of the inner structure thereof.
2. Description of Related Art
Conventionally, a small-sized portable apparatus, such as a TV, a receiver, a transmitter, or a pager (hereinafter collectively referred to as a "radio apparatus") is generally provided with an antenna of the loop type. The electrical operational theory and structure of this type of antenna is disclosed, for example, in the paper titled "A loop antenna for a small-sized portable radio set" published in "National Technical Report Vol. 19, No. 2, Apr. 1973". It also has been known that a loop antenna is provided at its central portion (in the longitudinal direction of the loop) with a reactance element such as a capacitor. In addition, when the radio frequency is relatively high, an inverted F type antenna is sometimes utilized for the radio apparatus.
One type of small-sized portable radio apparatus that has been developed is a wrist-mounted-type portable radio apparatus capable of being mounted around the user's wrist. Typical examples of the wrist-mounted-type portable radio apparatus are disclosed in U.S. Pat. No. 4,713,808 to Gaskill et al., wherein a loop antenna is provided, and in Japanese Laid-Open Patent Application No. 4-211522, wherein a short dipole antenna is provided.
However, when the loop antenna or the dipole antenna is utilized in the wrist-mounted-type portable radio apparatus, since the antenna is arranged in a state in which it contacts the user's wrist when the radio apparatus is mounted to the user, the directivity sensitivity of the antenna is degraded, and therefore satisfactory operation of the antenna cannot be expected.
The present inventor has proposed to use a slot antenna for a wrist-mounted-type portable radio apparatus and the like. The slot antenna is assembled in the wrist-mounted-type portable radio apparatus so that an electrically conductive plate forming a part of the slot antenna is accommodated in the main body of the radio apparatus, while another electrically conductive plate forming a remaining part of the slot antenna is arranged in wrist bands of the radio apparatus. These electrically conductive plates are connected to each other to form the slot antenna. In this wrist-mounted-type of portable radio apparatus having the slot antenna as described above, the inductance value of the antenna is defined by the circumferential length of the slot and the width of the electrically conductive plate of the antenna. The inductance value of the antenna is not affected by the length of the wrist bands when mounted around the user's wrist. Therefore, this antenna is advantageous because a high antenna gain can be maintained even if the size of the user's wrist varies. In addition, when mounted around the user's wrist, the slot antenna is arranged such that the slot faces outwardly, and therefore, the slot antenna advantageously is capable of detecting magnetic field contents generated around the body surface or wrist of the user.
However, because a wrist-mounted-type portable radio apparatus having a higher sensitivity has been demanded, there is a problem that a higher sensitivity of this type of radio apparatus cannot be expected by assembling the slot antenna therein in a conventional manner. The reasons are as follows:
First, in the wrist-mounted-type portable radio apparatus having the slot antenna, the radio signal receiving portion is electrically connected to the slot antenna. The connecting point defines the feed point of the antenna, which inevitably is located near the center portion of the slot antenna (relative to the longitudinal direction thereof). As a result, this feed point is situated in the main body of the radio apparatus, which has a variety of circuit components accommodated therein. An area around the center portion where the feed point is located also is the portion of the antenna where the radiated energy of the electrical waves is at a maximum. However, in the restricted space of the main body of the radio apparatus near this feed point, a variety of components comprised of dielectric materials, insulating materials, attenuation materials, electrically conductive materials or the like are arranged. If the components comprised of attenuation materials or electrically conductive materials are arranged along the radial direction of the electrical waves, the radiated energy may be insulated or degraded (damped) by these components, which causes deterioration of the sensitivity of the slot antenna.
Second, in the restricted thin space of the main body of the radio apparatus, circuit components that generate noise signals adversely affecting the transmitting and receiving sensitivities of the radio apparatus are arranged around the center portion of the slot antenna. These circuit components include digital-signal processing circuits, voltage up converted circuits (e.g., a DC/DC converter), and the like. The noise signals become transmission noises and radiation noises, which cause degradation of the sensitivity of the radio apparatus. When some types of circuit systems are adopted, the radiation noises may cause not only the radio itself, but also other radio apparatus to be disturbed.
Third, since the slot antenna is provided at its center portion with the reactance element (which is provided in order to resonate at a specific frequency), the portion of the slot antenna located in the main body of the radio apparatus is in a high impedance state. Thus, the value of Q of the antenna circuitry increases, so that the antenna impedance is adversely affected by the human body when the radio apparatus is mounted on the user's wrist. As a result, the most appropriate matching condition between the radio receiving section and the antenna section is deteriorated, whereby the transmitting and receiving sensitivity may be degraded. While it is possible to lower the value of Q by arranging an additional electrical element in the circuitry of the radio apparatus, this is not recommended because it prevents the down-sizing of the wrist-mounted-type portable radio apparatus.
Fourth, in the wrist-mounted-type portable radio apparatus, because the slot antenna is partially fixed to the wrist bands, it must be increased in mechanical strength. This means that an increase in the electrical resistance loss of the slot antenna must be accepted.